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Neurotransmitter receptors
A neurotransmitter receptor is a membrane receptor protein that interacts with neurotransmitters and mediate the flow of sinals through a neural network. A membrane protein interacts with the lipid bilayer that encloses the cell http://en.wikipedia.org/wiki/Membrane_protein and a membrane receptor protein interacts with a chemical in the cells external environment, which binds to the cell http://en.wikipedia.org/wiki/Receptor_protein . Membrane receptor proteins are particularly important in neuronal and glial (involved in neuronal transmission, but not technically neurons) cells, because they allow cells to communicate with one another through chemical signals. Neurotransmitter receptors send and receive signals that trigger an electrical signal that runs along the neuron and can be passed along a neural network, by regulating the activity of ion channels. It is important to appreciate that these receptors are not static structures but rather protein molecules produced within the neuron and they become temporarily embedded in the cell walls so that a portion of the molecule is exposed and acts as a binding site so that messenger molecules can attach themselves and so activate the receptor . The life cycle of individual receptors is only about 12-24 hours, after which they are deactivated through use or reabsorbed into the cell Most nerve cells have between 2000 and 3000 receptors on their surface }. Even where cells are specialized for one type of receptor molecule they usually are sensitive to other neurotransmitters. So for example a serotonin cell may have dopamine receptors as well as opiate receptors etc. The absolute number of receptors and the ratio of inhibiory to excitory receptors determines the reaction of the cell. Functional types Based on their structural and functional characteristics, neurotransmitter receptors can be classified into two broad categories: metabotropic and ionotropic receptors. In contrast to the latter, metabotropic receptors do not form an ion channel pore; rather, they are indirectly linked with ion-channels on the plasma membrane of the cell through signal transduction mechanisms. This class of receptors includes the metabotropic glutamate receptors, muscarinic acetylcholine receptors, GABAB receptors, and most serotonin receptors, as well as receptors for norepinephrine, epinephrine, histamine, neuropeptides (Austin, 2004; Purves et al., 2001) and dopamine (Zimmerberg, 2002). Ion channel coupled neurotransmitter receptors A neurotransmitter receptor can be paired directly with an ion channel, but most send signals indirectly though guanyl nucleotide-binding proteins or G proteins Interactions between neurotransmitters and neurotransmitter receptors are involved in a wide range of differing reactions from the cell receiving the signal, triggering anything from activation to inhibition. Desensitization and ligand concentration One additional characteristic of neurotransmitter receptors is that they are subject to ligand-induced desensitization: That is, they can become unresponsive upon prolonged exposure to their neurotransmitter. Neurotransmitter receptors are present on both postsynaptic neurons and presynaptic neurons with the former being used to receive neurotransmitters and the latter for the purpose of preventing further release of a given neurotransmitterhttp://web.indstate.edu/thcme/mwking/nerves.html#table. In addition to being found in neuron cells, neurotransmitter receptors are also found in various immune and muscle tissueshttp://crisp.cit.nih.gov/Thesaurus/00005559.htm. Many neurotransmitter receptors are categorized as a serpentine receptor or G protein-coupled receptor because they span the cell membrane not once, but seven times. Neurotransmitter receptors are known to become unresponsive to the type of neurotransmitter they receive when exposed for extended periods of time. This phenomenon is known as ligand-induced desensitizationhttp://web.indstate.edu/thcme/mwking/nerves.html#table or downregulation. Relationship to the action of psychiaric drugs Most psychiatric medications do not act upon ionic receptors. Although the minor tranquilizers and benzodizepines do act by opening the chloride ion channels and for this reason are fast acting. However most are slow acting because they work through metabotropic mechanisms that alter the cells internal machinery over time through acting on the second messenger system. Known neurotransmitter receptors (table2)ed. Kebabain, J. W. & Neumeyer, J. L. (1994). "RBI Handbook of Receptor Classification" *Adrenergic α1A, α1b, α1c, α1d α2a, α2b, α2c, α2d β1, β2, β3 *Dopaminergic D1, D2, D3, D4, D5 *GABAergic GABAA, GABAB1a, GABAB1δ, GABAB2, GABAC *Glutaminergic NMDA, AMPA kainate, mGluR1, mGluR2, mGluR3, mGluR4, mGluR5, mGluR6, mGluR7 *Histaminergic H1, H2, H3 *Cholinergic Muscarinic: M1, M2, M3, M4, M5 Nicotinic: muscle, neuronal (α-bungarotoxin-insensitive), neuronal (α-bungarotoxin-sensitive) *Opioid μ, δ1, δ2, κ *Serotonergic 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT4, 5-HT5, 5-HT6, 5-HT7 *Glycinergic Glycine See also * Autoreceptor * Catecholamines * Cholinergic Agonists and Antagonists * Metabotropic * Neuromuscular transmission * Receptor desensitization has been previously modeled in the context of a two-state mathematical model (see this link http://www.bio-balance.com/Graphics.htm) * Synaptic Transmission Notes and references External links * Category:Receptors